Abstract: The depth and scale of potash mines is currently increasing. Backfill materials with higher homogeneity and greater fluidity property are required to prevent blocking and ensure that filling materials are transported safely to the underground mines. The production of low-cost backfill materials that meet both strength and transportability requirements is therefore extremely important. A potash filling material with steel slag as a cementing agent was prepared to solve the problems caused by potash tailings and brine water in potash mines, such as environmental pollution, resources waste, and potential safety issues. This was also done with the aim of improving the flow and strength properties of potash filling materials by using steel slag as binder because its hydration reaction is slow and durable. The results indicated that the fluidity and late strength met requirements. It was also initially proved that the steel slag fineness and curing temperature had a significant influence on the performance of the filling material. The fluidity of the filler was greater than 200 mm in 8 h and the compressive strength reached 2 MPa in 28 days, which satisfied the performance requirements for the filler. This article mainly focuses on the analysis of the curing mechanism from a microscopic point of view. The results of X-ray diffraction (XRD), scanning electron microscopy with energy dispersive spectrometer (SEM-EDS), thermogravimetry/differential thermal analysis (TG/DTA), and Fourier-transform infrared spectroscopy (FTIR) show that the products of the hydration reactions are primarily C-S-H gels, hydrocalumite (or Friedel's salt), and brucite. They are interspersed with each other so that the density and strength improve. The results show that isomorphous substitution occurs in the microscopic structure of the layered hydrocalumite and hydrotalcite. The Ca²⁺, Mg²⁺, Fe²⁺, Fe³⁺, Al³⁺, and Si⁴⁺ can replace each other and enable OH^-, Cl^-, and H₂O molecules to be easily absorbed into the interlayer. In this way, impurity ions can be stabilized and stability of the filling material can be improved. The results of this study provide preliminary evidence that steel slag powder can be used as a binder in potash backfill material.